1. Field of the Invention
The present invention relates generally to the field of laser velocimetry, and more specifically, to a laser doppler velocimeter (LDV) which uses frequency shifting and a single channel digital frequency processor to effect multi-velocity component measurements.
2. Description of the Related Art
LDVs are now in common use for flow measurements during, for example, wind tunnel testing of various structures. There are many types of known LDVs, but most operate off similar basic principles.
Dual beam LDVs use optics to split a laser beam into two beams and to focus the beams to cross at a point where measurements are to be made. Due to wave interference, a fringe pattern is formed at the beam intersection. A second lens assembly, functioning as a receiver, is focused on the fringe region to collect light scattered from seed particles crossing the fringes. The collected light is fed to a photodetector that is used as the input to an electronic signal processor that measures modulated frequency.
In a dual beam system the frequency from the scattered light from the split beams is directed onto the surface of the photodetector where it is mixed, and a difference in frequency between the two beams will result.
Flow velocity can be derived from the modulated frequency, which can be expressed as 2.mu..sub.x sin .beta./.gamma.
where
.mu..sub.x =velocity parallel to the plane of the two beams and perpendicular to a bisector of the beams; PA1 .beta.=angle between one of the beams and the bisector, and PA1 .gamma.=wavelength of the laser light.
An example of a signal processor for a LDV is described in U.S. Pat. No. 4,786,168, issued to Meyers et al. on Nov. 22, 1988. A processor is described therein for measuring the signal frequency within a signal burst. A photodetector converts an optical signal, composed of scattered light emanating from the fringe pattern, into an electronic signal. The electronic signal is composed of a collection of Poisson distributed photo-electrons whose average occurrence rate is proportional to the instantaneous light intensity at the photocathode. As the intensity increases from the photon resolved regime, the additional photon arrivals within the response time add voltage to the output signal. Eventually, the signal approaches a Gaussian-shaped signal burst containing the oscillation frequency. The frequency is multiplied by the distance between adjacent fringes to yield the velocity of the particle.
The aforementioned processor of Meyers et al. converts the input signal to digital and shifts it into shift registers. A signal integration circuit determines when a signal burst has been captured by the shift registers and transfers the contents to data latches, whereupon the data is processed by bandpass filters, square law detectors, burst counters and a signal processor to determine the frequency of the signal.
W. Farmer and J. Hornkohl describe a LDV which measures two-vector components of velocity in an article entitled "Two-Component, Self-Aligning Laser Vector Velocimeter", Applied Optics, vol. 12, no. 11 (Nov. 1973). The LDV, schematically illustrated in FIG. 1, includes a laser 10 outputting a beam to a two-dimensional Bragg cell 12. The Bragg cell 12 is actually two Bragg cells in a common housing arranged so that the center lines of the cells are coincident and orthogonal. This arrangement allows the Bragg cell 12 to be both a beam splitter and a frequency shifter. The crystal oscillators of the Bragg cell 12 are driven at two different frequencies for detection separation of the respective velocity components.
The Bragg cell 12 produces four output beams, three of which are diffracted beams and the fourth is the original input beam. The diffracted beams can be independently shifted up or down in frequency. The beams are focused by a lens 14 to cross simultaneously at a point. Scattered light is collected by a lens system 16 and focused on a photomultiplier 18 and the output signal therefrom is processed by signal processors 20 (only one shown) which use band-pass filters 21 to separate the carrier frequencies for the respective velocity components.
Other methods of obtaining simultaneous multi-velocity component LDV measurements involve, instead of frequency shifting, the use of a different laser wavelength for each velocity component. Generally, multiple photodetectors and signal processors are used with narrow-band light filters placed ahead of the photodetectors. The use of multiple processors, either counter-type or digital frequency-type in LDV measurements, with one provided for each velocity component, is fairly common.
It is generally accepted in the laser doppler velocimeter field that a separate signal processor channel is needed for each velocity component signal under sparse seeding conditions. High speed digital counters facilitate this procedure by measuring the time between zero crossings of signals produced from individual particles passing through the LDV sensing volume. However, the requirement for multiple processing channels and photo-detectors makes the overall set up of an LDV using different wavelength (color) lasers complicated and expensive.